The proposed experiments are designed to detect, at the level of the single cell, the alterations that follow physiologically meaningful stimulation of vasopressinergic and oxytocinergic neurons. Much is known about these neurons, but previous investigations studied global alterations in levels of mRNA and proposes. Individual neurons within apparently homogeneous anatomical nuclei differ in terms of their afferent and efferent connectivity, and in terms of peptide co-localization. Hence, in approach to neuroendocrine function should be employed tbat can detact if hormone response is restricted to specific vasopressin or oxytocin cells. Utilizing quantitative in situ hybridization to determine relative amounts of vasopressin and oxytocin mRNA levels, I will examine whether all hormone-synthesizing cells within a hypothalamic cell group response to certain physiological challenges, or whether specific subgroups of cells mediate hormone response. Based upon investigations of cell connectivity, and other factors described below, it is hypothesized that hormone response is mediated by specific vasopressinergic and oxytocinergic cells. Combining in situ hybridization with immunocytochemistry for peptide co-localization, and with tract-tracing techniques, I hope to determine the dynamic, anatomical pattern of response in vivo, of these neural systems, and the basis for elucidating the salient characteristics of those cells that specifically mediate response to homeostatic challenge. In addition to providing a model for how the function of single neurons is integrated within the context of a neuroendocrine system, these studies may have clinical relevance. First, disease processes that disrupt the regulation of synthesis, transport, or release of vasopressin can precipitate inappropriate compensation of bodily fluid needs. Proposed experiments that involve water balance and how the neurohypophysial system responds are relevant. Second, vasopressin plays a role in hypertension and stress, and study of neuronal response at the single cell level will provide information regarding differential response in autonomic or neurohaemal-mediating vasopressinergic neurons compared to neurohypophysial neurons. Finally, oxytocin gene expression is an almost unexplored matter that may be clinically relevant to oxytocin's role in pregnancy, lactation, and stress.